Display device including a display element of dot matrix type and a drive method thereof

Abstract

A display device includes a display element of a dot matrix-type having a display material with memory properties, a drive circuit that drives a pixel of the display element and a control circuit that controls the drive circuit, the control circuit includes an initialization step for applying a voltage pulse to initialize a pixel to be rewritten to bring about an initial gradation state and a gradation step for applying a voltage pulse to change the gradation state of a pixel, and the gradation step includes a plurality of sub-steps having a plurality of execution times and in the plurality of sub-steps, an alternating-current voltage pulse is formed in a pixel to be rewritten and the period of the alternating-current voltage pulse is varied in accordance to a gradation to be written within the range of the sub-step.

1 . A display device comprising:
 a display element of dot matrix type including a display material with memory properties;   a drive circuit that drives a pixel of the display element; and   a control circuit that controls the drive circuit, wherein:   the control circuit comprises: an initialization step for applying a voltage pulse to initialize a pixel to be rewritten to bring about an initial gradation state; and a gradation step for applying a voltage pulse to change the gradation state of a pixel; and   the gradation step comprises a plurality of sub-steps including a plurality of execution times and in the plurality of sub-steps, an alternating-current voltage pulse is formed in a pixel to be rewritten and the period of the alternating-current voltage pulse is varied in accordance to a gradation to be written within the range of the sub-step.   
   
   
       2 . The display device according to  claim 1 , wherein
 the plurality of sub-steps are performed in different frames.   
   
   
       3 . The display device according to  claim 2 , wherein
 the plurality of sub-steps are performed in the frames in the order of decreasing number of gradations to be written.   
   
   
       4 . The display device according to  claim 3 , wherein
 the plurality of sub-steps are terminated temporarily at the point of time when wiring in a predetermined sub-step is completed.   
   
   
       5 . The display device according to  claim 1 , wherein the alternating-current voltage pulse is formed only in one cycle during the period of selection of a scan line. 
   
   
       6 . The display device according to  claim 5 , wherein
 the alternating-current voltage pulse includes a positive polarity phase and a negative polarity phase of the same length.   
   
   
       7 . The display device according to  claim 1 , wherein the drive circuit comprises a driver IC that outputs two values for the pixel of the selected scan line. 
   
   
       8 . The display device according to  claim 1 , wherein
 the period of the alternating-current voltage pulse is determined by data to be supplied to a segment driver IC.   
   
   
       9 . The display device according to  claim 1 , wherein
 the execution time of the sub-step is controlled by switching the latch periods of the segment driver IC.   
   
   
       10 . The display device according to  claim 1 , wherein
 the display material is liquid crystal that forms a cholesteric phase.   
   
   
       11 . The display device according to  claim 10 , wherein
 the initial gradation state in the initialization step is a planar state, the gradation state in the gradation step is a state where the planar state and a focal conic state coexist in a mixed condition, and a halftone value is determined by a coexistence ratio.   
   
   
       12 . The display device according to  claim 11 , wherein
 a difference of the period of the alternating-current voltage pulse from that of a neighboring gradation is larger when the coexistence ratio of the focal conic state is large than when the coexistence ratio of the focal conic state is small.   
   
   
       13 . The display device according to  claim 1 , wherein
 the display device comprises a laminated structure in which a plurality of display elements that exhibit a plurality of kinds of reflected light are stacked.   
   
   
       14 . The display device according to  claim 1 , wherein
 in the display device, the operation timing at which the plurality of display elements are brought into the initial gradation state is different from the operation timing at which the state is brought into a gradation state other than the initial gradation state.   
   
   
       15 . A driving method of a display device, the device comprising:
 a display element of dot matrix type including a display material with memory properties;   a drive circuit that drives a pixel of the display element; and   a control circuit that controls the drive circuit, wherein:   the control circuit comprises an initialization step for applying a voltage pulse to initialize a pixel to be rewritten to bring about the initial gradation state and a gradation step for applying a voltage pulse to change the gradation state of the pixel; and   the gradation step comprises a plurality of sub-steps including a plurality of execution times, in the plurality of sub-steps, an alternating-current voltage pulse is formed in a pixel to be rewritten, and the period of the alternating-current voltage pulse is varied in accordance with a gradation to be written within the range of the sub-step.   
   
   
       16 . The driving method according to  claim 15 , wherein
 the plurality of sub-steps are performed in different frames.   
   
   
       17 . The driving method according to  claim 15 , wherein
 the alternating-current voltage pulse is formed only in one cycle during the period of selection of a scan line.   
   
   
       18 . The driving method according to  claim 17 , wherein
 the alternating-current voltage pulse includes a positive polarity phase and a negative polarity phase of the same length.   
   
   
       19 . The driving method according to  claim 15  wherein
 the display material is liquid crystal that forms a cholesteric phase.   
   
   
       20 . The driving method according to  claim 19 , wherein
 the initial gradation state in the initialization step is a planar state, the gradation state in the gradation step is a state where the planar state and a focal conic state coexist in a mixed condition, and a halftone value is determined by a coexistence ratio.